Propeller blade position controller

ABSTRACT

System for reducing vibrations in the cabin of an aircraft driven by two or more propellers each having n blades (n being an integer equal or larger than 2) of which the relative phase angle can be adjusted, said system comprising synchrophase means for adjusting said relative phase angle such that a predetermined relative phase angle is maintained. The system furthermore comprises means supplying information about the vibration level at one or more positions within the cabin or thereto related information and for generating a thereto corresponding level signal, and a control unit which in response to said level signal supplies an error signal to the synchrophase means causing the synchrophase means to adjust the blades of the various propellers such that a new relative phase angle is maintained which differs m.(2π/n) radians (m being an integer) from the previous relative phase angle.

The invention relates to a system for reducing vibrations in the cabinof an aircraft driven by two or more propellers each having n blades (nbeing an integer equal or larger than 2) of which the relative phaseangle can be adjusted, said system comprising synchrophase means foradjusting said relative phase angle such that a predetermined relativephase angle is maintained.

A source of discomfort for passengers and crew of propeller-drivenaircraft is the action of the various engines and propellers generatingannoying vibrations and dominant noise in the cabin and cockpit. Apropeller with a rotational speed N and a number of blades n, generatesexcitation forces having frequencies of N, Hz and nN Hz with harmonics.Vibrations of this nature have regular and irregular patterns. In orderto reduce the vibration and internal noise to an appreciable level forcrew and passengers, modern propeller driven multi engined aircraftutilize all kind of energy absorbing, vibration isolating and reactiveforce devices to reduce the transmission of mechanical vibrations fromthe propulsion units to the fuselage.

For example, isolators are often mounted in the connection betweenengine and engine support to isolate the engine/propeller combinationfrom objectionable dynamic foundation displacements, and/or isolate theaircraft structure from objectionable dynamic forces from theengine/propeller combination. Such apparatus include elastomeric andmetal spring elements for dampening vibration in one or more directions.

Yet another method to reduce propeller harmonics in particular is theuse of propeller synchrophase systems. Examples of such systems aregiven by U.S. Pat. Nos. 2,847,617; 2,877,855; 2,878,427; 3,007,529;3,589,832; 4,245,955; 4,653,981; 4,659,283 and 4,703,908; Europeanapplication EP 0,322,343; Canadian Patents 0,664,628 and 0,472,689 andthe UK Patent 2,211,635. The object of these prior art systems forpropeller synchrophasing is to maintain an accurate positional or phasicagreement between one blade of a first propeller, which is named themaster, and any blade of the other propeller(s), which are named theslaves.

The complex factors determining the human response to vibrations and thepaucity of consistent quantitative data concerning man's perception ofvibration and his reactions to it, are difficult to translate ingenerally recognizable criteria of comfort or discomfort. However, whenthe intensity, frequency and duration of the vibration at severallocations in the cabin, and also consequences of such vibrations likevisible vibration of passenger seats or tables are judged by a varietyof passengers in comfort or discomfort, acceptance criteria can bederived. From a survey of many aircraft an acceptable maximum level ofthe energy of the vibration at several locations in the cabin wasestablished.

During first flights of Fokker 50 series aircraft, which is a twinengined propeller-driven passenger aircraft, it appeared that some ofthe aircraft would expose future passengers to uncomfortable vibrations.With the help of above mentioned acceptance criteria, the influence ofmodified vibration engine isolators was investigated. Also investigatedwas the influence of minimizing the propeller mass unbalance. Results ofboth investigations indicated that the vibration level could be reducedfor the given new-built aircraft. However, after delivery of theaircraft to the operator maintenance of the aircraft would change theconfiguration, for example by the replacement of a propeller. Thereforethe cause of vibrations and noise in the cabin which affect passengercomfort in a negative way, had to be solved more fundamentally.

A further reason to proceed investigations was the observation thatdespite modified isolators and minimized unbalance, during flight afteran engine shut down and relight a period of sufficiently low energylevel of vibrations before the shut-down was succeeded by a period witha higher level after the relight, and vice versa. Continued measurementsduring flight tests whereby the phase relationship between the twopropellers of the aircraft was controlled by the propeller synchrophasesystem, showed a relation between the vibration level and the changes ofthe position of the blades of the starboard propeller with respect tothe blades of the port propeller. From these observations distinctionsin detail were made between the vibration and noise level in the cabinbefore and after the synchrophase system had readjusted upon adisturbance the required phase relationship between both propellers.

By measurements of the relation between the vibration levels in thecabin and the relative position of the blades of the right handpropeller versus the left hand propeller, it was found that when theposition of a particular blade of one propeller and the position of aparticular blade of the other propeller were synchrophased, thevibration level in the cabin showed a minimum. Also the contrary wasmeasured, namely that a maximum vibration level could occur bysynchrophasing two particular other blades. The findings were worked outin an add-on system of the prior art propeller synchrophase system toarrange that the add-on system in conjunction with the synchrophasesystem selects automatically the pair of blades from the propellers ofthe aircraft that gives the minimum vibration level in the cabin, andthrough that an improved comfort for crew and passengers. The add-onsystem does not effect the above mentioned specific function of thesynchrophase system itself.

More specifically the invention provides a system for reducingvibrations in the cabin of an aircraft driven by two or more propellerseach having n blades (n being an integer equal or larger than 2) ofwhich the relative phase angle can be adjusted, said system comprising

synchrophase means for adjusting said relative phase angle such that apredetermined relative phase angle is maintained, characterized in thatthe system furthermore comprises

means for supplying information about the vibration level at one or morepositions within the cabin or thereto related information and forgenerating a thereto corresponding level signal, and

a control unit which in response to said level signal supplies an errorsignal to the synchrophase means causing the synchrophase means toadjust the blades of the various propellers such that a new relativephase angle is maintained which differs m.(2π/n) radians (m being aninteger) from the previous relative phase angle.

The system according to the invention offers the possibility to phaselock both propellers such that not only a predetermined phase relationis maintained between the propeller blades of both propellers ingeneral, but more specifically a predetermined phase relation ismaintained between a particular blade of one propeller and a particularblade of another propeller.

The invention will be described in more detail in the following part ofthe description wherein reference is made to the attached drawings.

FIG. 1 is a block diagram illustrating the elements of a propeller bladeposition regulator according to the present invention.

FIG. 2 is a block diagram showing in more detail than FIG. 1 anembodiment of a propeller synchrophase system according to theinvention.

FIG. 3 illustrates schematically the possible angular positioncombinations of two 6-blade propellers in which the same phase relationbetween the propeller blades in general is maintained.

FIG. 4 shows a schematic curve of the change of the vibration level of ahard point in the cabin as function of the six successive bladecombinations of two six-blade propellers.

FIG. 5 is a block diagram showing in more detail than FIG. 1 anotherembodiment of a similar phase system according to the invention.

As shown in FIG. 1 the propeller blade position control system 1consists of a conventional synchrophase system 2, a source of vibrationlevel data 3, providing vibration level data about one or more positionsof the aircraft structure, and a control unit 4. The vibration leveldata source may comprise one or more transducers or pick-ups and adevice or devices to amplify the amplitude or level of the vibrationsignal. The sensed signals of the measuring equipment are supplied vialine 5 to the control unit 4. The unit 4 analyses the signals inagreement with one of a number of possible algorithms, some of whichwill be described in more detail. If for instance the signal exceeds apredetermined level during a predetermined time period, the control unit4 generates a command signal which is led through line 6 to thesynchrophase system. The system 2 will change the existing combinationof blades which are synchrophased whereby a momentary out-of-phaseposition of the propellers shall be as short as possible.

In other embodiments the vibration level data source comprises a memoryin which information is stored about the particular angular phaserelation of both propellers which results into a minimum vibration levelin the cabin. Specific examples of both types of embodiments will beexplained hereinafter.

FIG. 2 illustrates schematically a propeller blade position controlleraccording to the invention, destined for a twin-propeller aircraft. Toindicate the correspondence between the general block diagram in FIG. 1and the more detailed block diagram in FIG. 2 the main sectionsillustrated in FIG. 1 are again illustrated in dashed lines in FIG. 2.

In FIG. 2 the engines 11 and 21 are connected to the respectivepropellers 12 and 22 through their hydromechanical propeller blade pitchcontrol mechanisms which are separately indicated by 13 and 23. Therotational speed of each engine-propeller combination is controlled by acontroller indicated with reference numbers 14 and 24 respectively. Bothcontrollers 14 and 24 receive an input signal RPM from the terminal 19representing the desired nominal number of revolutions per minute towhich the propellers should be adjusted. The blade passing frequency isregistered by the detectors 15 and 25 respectively. Both detectorsdeliver their signals to the respective controllers 14 and 24 in whichthese signals are compared with the ROM-signal and in which, ifnecessary, control signals are generated to be supplied to therespective blade pitch control mechanisms 13 and 23. Further bothdetectors deliver their signals to a phase comparator 16. If there is ameasurable difference in phase angle between both propellers, thecomparator 16 generates a phase difference dependent signal which issupplied to a processor 17. The processor 17 receives furthermore aphase offset signal from terminal 20. If the measured phase differencedeviates from the value given by the phase of the input 20, aconditioned error signal is delivered to the speed controller 24. Thespeed controller 24 will respond to the error signal and will cause amomentary increase or decrease in the speed of propeller 22 such thatthe phase difference of the signals measured by the detectors 15 and 25is adjusted to the offset value received on terminal 20. The RAM-signalon terminal 19 and the phase offset-signal φ-offset on terminal 20 aregenerated by other systems in the aircraft as is known to the expert.

The components 11-25 of the synchrophase system described so far are infact completely known from the prior art. Further details about thefunctioning of this part of the system can be found in the abovementioned publications.

The system illustrated in FIG. 2 comprises furthermore within the dashedframe 3 a number of vibration detectors 30, 31, 32 and 33 destined tomeasure the vibration level at different positions in the cabin of theairplane. (The actual number of detectors can be selected by the user).The signals generated by these vibration detectors are supplied to aprocessor 34 in which the received signals are processed according to apredetermined algorithm. The processor 34 has the same function as thecontrol unit 4 in FIG. 1. If the processor 34 decides that the momentaryangular position combination of the blades of both propellers stillresults in a general vibration level in the cabin above a predeterminedreference level, then the processor 34 will supply a control signal tothe processor 17 causing said processor to provide a signal to thecontroller 24. The controller 24 will respond to the signal and willcause a momentary increase or decrease in the speed of propeller 22 suchthat the propellers will become phase locked in another angular positioncombination.

FIG. 3 illustrates the various angular position combinations of the twopropellers whereby the same mutual phase relationship is maintained.Each propeller has six blades and one of the blades, blade 40 ofpropeller 1 and blade 41 of propeller 2, is shaded in FIG. 3 and willact as reference blade. It will be clear from FIG. 3 that in each of thesix possible angular position combinations both propellers have the samephase relation although, if the blader are considered as differentindividual components, the six illustrated combinations are different.All prior art synchrophase systems try to maintain a predetermined phaserelation between both propellers without looking at the individualblades of the propellers. The system according to the underlyinginvention however takes the individual blades into account. In otherwords, the prior art systems try to maintain a predetermined phase angleφ between the propeller blades whereby

    0<φ<2π/n (n being the number of blades),

whereas the herein described system will try to maintain a predeterminedphase angle φ between the propeller blades whereby

    0<φ<2π.

The advantages of the system according to the invention can be madeclear with reference to FIG. 4 which shows an example of a recording ofthe vibration level measured with a sensor at a location at the seatrail in the floor of the cabin of the Fokker 50 which was used for theexperiments. As already indicated above the Fokker 50 has two 6-bladepropellers and as explained above a predetermined phase angle betweenboth propellers can be maintained with six different angular positioncombinations of the blades of both propellers. Shown is the relationbetween the vibration level and the six combinations between the bladesof the first propeller and the blades of the second propeller. The sixcombinations are indicated by reference numbers 51-56 in FIG. 3 and thesame reference numbers are used in FIG. 4. The recording shows a minimumlevel for the preferable blade combination 52 and a maximum level forcombinations 54 and 55.

A practical embodiment of the system according to the invention mayoperate as follows. After the synchrophase subsystem 2 has reached aphase lock situation wherein both propellers are locked in anyone of thesix possible combinations the measuring circuit 3 starts measuring thevibration level. The measured vibration level data is stored in a memoryin the processor 34 and said processor generates thereafter a controlsignal to shift the position of the blades of the controlled propeller2π/n radians (=360/n degrees) with respect to the other propeller. Againthe vibration level is measured and stored in the memory of theprocessor 34 whereafter a further control signal will be applied to theprocessor 17 to cause a further 2π/n radians shift of the propellerblades. In this way the vibration level is measured for all sixpropeller blade combinations. Based on the stored data the processor 34finally generates the ultimate control signal causing a phase shift ofm(2π/n) radians whereby m is selected by the processor 34 between 0 and5 such that the combination with minimum vibration level (combination 52in the above example) will be selected and maintained.

In the above described example it is more or less assumed that only onevibration level sensor was active within the vibration level data source3. In practical embodiments, however, a number of sensors will beinstalled within the cabin and the signals delivered by those sensorswill be combined according to a predetermined algorithm. In the exampleillustrated in FIG. 2 there are 4 sensors which are preferably installedat various representative locations within the cabin. It is for instancepossible to determine the sum or the average of the various signals,which sum or average is used in the processor 34 as criterium to decideon. However, it is also possible to use a priority scheme in which thesignal from predetermined sensors are given a higher weight than thesignals from other sensors. The development of suitable algorithms forprocessing a number of signals derived from various sensors isconsidered within reach of the expert in this field.

In another embodiment of the invention the system is engaged as soon asafter the take-off of the aircraft the synchrophase system is activated.The processor 34 in control unit 4 compares the sensed momentaryvibration level in the cabin with a prefixed reference value. In casethere is a substantial difference in value, the processor 34 presents acommand signal proportional to 2π/n radians (360/n degrees) to theprocessor 17. In reaction the synchrophase system will change the speedof propeller 22 by fining the blade pitch, such that the position of theblades of that propeller will be shifted 360/n degrees with respect tothe blades of the other propeller 12. The time the shift will take ismainly determined by the size and the weight of the propeller. When thestandard phase off-set between the propellers is re-adjusted and thevibration level in the cabin is not yet equal or below the prefixedreference value, the described action by the processor 34 is repeateduntil the first blade combination for which the vibration level in thecabin is equal or below the maximum level is established.

In yet another embodiment of the invention, which is separatelyillustrated in FIG. 5, the processor 34 is coupled to a memory 34a inwhich the combination of the position of propeller blades is storedwhich gives an minimum vibration and noise level. The respectiveinformation can be obtained for instance in the above described mannerduring a test flight. Furthermore the system comprises two additionalsensors 35 and 36 destined to detect the actual position of thereference blades 40 and 41 respectively. Both sensors supply a signal tothe processor 34. As soon as the synchrophase system has reached astable condition in which propeller 22 is phase locked to propeller 12the processor 34 determines which of the six possible angular propellerblade combinations is momentarily selected by the synchrophase system.This is done by evaluating the signals received from the additionalsensors 35 and 36. Thereafter the processor 34 calculates the differenceangle between the momentary combination and the preferred combinationand generates a corresponding error signal to the processor 17 causingthe synchrophase system to lock both propellers in the preferredcombination. Dependent on the actual calculated phase difference theerror signal may result either into a momentary decrease or in amomentary increase of the rotational speed of propeller 22 to reach thepreferred combination. In the case the control unit finds that thevibration and noise level in the cabin exceeds the level for comfort, acommand signal is calculated which shifts the position of one propellerwith respect to the other propeller at one go from the "uncomfortposition" to the "comfort position".

In yet a further embodiment of the invention the add-on system isengaged as soon as after the take-off of the aircraft the synchrophasesystem is activated. Said control unit 4 determines the combination ofblades for minimum vibration level in the cabin and measures thevibration level for all other combinations. An alert limit is calculatedfrom the average of the maximum and the minimum value and the minimumvalue. The next step of the control unit is to generate a first commandsignal to the synchrophase system if the momentary vibration level isabove the alert limit. Upon the first command signal, the synchrophasesystem shall shift the blade combination to the combination for minimumvibration. Hereafter the control unit continuously compares themeasured, momentary vibration level with the alert limit. If this limitis exceeded, a second command signal is generated. And so on. Thecontrol unit will verify from time to time whether the selected bladecombination for minimum vibration level still provides the minimumvibration level. If this is no longer the case, the unit will repeat theselection procedure for the best blade combination.

An experimental embodiment of the above described blade matching systemwas tested in an Fokker 50 series aircraft. This test system hadfacilities to operate in three different modes, i.e. a step by stepmode, a continuous mode, and an auto minimum mode. Before the systemstarts functioning in one of the selected modes the system checks firstof all if the number of revolution per minutes of both propellers (theFokker 50 series aircraft has two propellers) are within a predeterminedwindow, which window is specific for cruise flight conditions.Thereafter the cockpit crew selects one of the above mentioned modeswith the following result:

1) Step by Step Mode.

By selecting this mode the system is instructed to carry out a one bladeshift operation. After a predetermined stabilization period (of forinstance 10 seconds) the propellers are stable in their new mutualrelation and if necessary the crew may operate the system once more tocarry out a further blade shift operation.

2) Continuous Mode.

In this mode the crew has the possibility to start a sequential bladeshift operation whereby with predetermined intervals of for instance 30seconds a blade shift operation is carried out. This continuous mode isfor instance useful for measuring the vibration level at the differentrelative phase angles and accumulating the respective measured values ina memory for further use.

3) The Auto Minimum Mode.

In this mode a number of sequential steps are carried out:

1. First of all the system checks if the number of revolutions perminute is within the predetermined window, which window isrepresentative for normal cruise conditions. If this is true then step 2is carried out.

2. The system waits for a predetermined period of for instance 10seconds to be sure that the propellers have stabilized.

3. With predetermined intervals of for instance 10 seconds the vibrationlevel is measured at predetermined different positions in the cabin ofthe airplane. The measured values are stored in a memory.

4. After three samples measured in step 3. The average vibration levelis calculated and the calculated results are stored in a memory.

5. If the average vibration level, calculated in step 4, is above apredetermined threshold level then step 6, is carried out. Otherwise theprocess goes back to step 3.

6. With predetermined time intervals a number of blade phase shiftinstructions is provided to the system to carry out an equal number ofblade shift operations. The length of the above mentioned time intervalsis sufficient to provide for a first interval section in which theactual blade shifting is carried out, a second interval section in whichthe engines are enabled to stabilize and a third interval in which thevarious samples are measured from which an average value is calculated.

7. The calculated average sample values are stored in a memory.Altogether values are stored for each of the different relative phaseangles.

8. The minimum of all stored average values is calculated.

9. A number of blade phase shift operations is carried out, the numberbeing selected such that the propellers are adjusted to such a phaseangle that the minimum vibration level, calculated in step 8, isobtained.

10. With predetermined time intervals of for instance 10 seconds thevibration level is sampled.

11. The average value of three samples taken in step 10, is calculated.

12.

a. If the average value, calculated in step 11, is smaller than or equalto a first limit value then the process switches back to step 10.

b. If the average value, calculated in step 11, exceeds the minimumlevel calculated in step 8, with a predetermined tolerance value thenthe process switches back to step 6.

c. If the calculated average value is larger than a second thresholdvalue after a predetermined time interval of for instance 5 minutes,then the process switches back to step 6.

d. If the average value calculated in step 8, is larger than a furtherlimit value after a further time interval of for instance 15 minutesthen the process switches back to step 6.

13. Go to step 10.

Although above various embodiments are described in which it is assumedthat the aircraft has two propellers it will be clear that the inventioncan be applied also in case another number of propellers is used. Ingeneral the inventive system can be applied to an aircraft having mpropellers each comprising n blades. The number of possible angularposition combinations can be expressed in general as n^(m-1). For anaircraft with four 6-blade propellers the number of possiblecombinations would be 6^(a) =216. In such a case it would be very timeconsuming to measure the vibration level for each of these combinations.Therefore it is preferable under those circumstances to use anembodiment in which the momentary position of all propellers is detectedand compared with the desired preferred combination which is stored in amemory whereafter calculated error signals are supplied to thecontroller of the synchrophase system to lock each propeller in thepreferred combination.

I claim:
 1. A method of reducing vibrations in the cabin of an aircraftdriven by a plurality of propellers each having n blades, n being aplural integer, wherein said propellers have a relative phase anglewhich can be adjusted, the method comprising the steps of:a) measuringthe vibration level at one or more positions within the cabin andgenerating a vibration level signal corresponding thereto; b) evaluatingan error signal in response to said vibration level signal; c)optimizing the relative phase angle of the propellers in response tosaid error signal to a first value at which said measured vibrationlevel is reduced; d) shifting said propellers relatively through 2π/nradians to a new relative phase angle where said propellers aresubstantially in phase and have new relative blade positions; e)optimizing said new relative phase angle in response to a correspondingvibration level error signal to a second value at which said measuredvibration level is reduced; and f) selecting between said first andsecond values for improved vibration reduction in the cabin.
 2. A methodaccording to claim 1, wherein said propellers each have m blades andwherein steps d) and e) are repeated to generate m optimized relativephase angle values for each of said m blades, step f) being performed toselect between all said m values.
 3. A method according to claim 2,wherein m is six.
 4. A method according to claim 1 which comprises:comparing said vibration level signal with a prefixed reference valueduring flight conditions;evaluating said error signal according to thedifference between said vibration level signal and said prefixedreference value; performing step d) when said error signal exceeds apredetermined threshold value; and measuring the vibration level again;and repeating the former steps until said error signal does not exceedsaid threshold value anymore.
 5. A method according to claim 4 performedon an aircraft having four propellers wherein said propellers each havesix blades and wherein steps d) and e) are repeated to generate sixoptimized relative phase angle values one for each of said six blades,steps f) being performed to select between all said six values.